October 22, 2016
Combustion optimization has become an absolute necessity for all combustion processes. Optimization improves efficiency, reduces environmental impact, minimizes maintenance requirements, delays maintenance shutdowns and reduces forced outages.
Poor control of the combustion process will cause problems such as soot formation, emission problems, hot spots and/or corrosion in the flue ducts. These issues will increase maintenance expenditures and reduce the life cycle of the plant, not to mention the fact that efficient combustion saves on fuel costs.
The most efficient combustion is achieved when the exact amount of fuel is mixed with the exact amount of air in precise conditions. While this is the targeted operating scenario, it is never achieved, and any variation in the fuel to air mix or a quick change in the furnace condition can cause incomplete combustion.
Under normal boiler processes, if there is incomplete combustion of hydrocarbon fuel, the exhaust gases are water, carbon dioxide, carbon monoxide, hydrogen, oxygen, plus aldehydes. If Sulphur (present in many fuels) is added, plus nitrogen and oxygen from the air, the list of gases in the exhaust will then include Sulphur Oxides (SO, SO2), Nitrogen Oxides (NO, NO2) plus Nitrogen. Nitric Oxide and Nitrogen Dioxides (generally called NOx) are the main pollutants produced by combustion. Thermal NOx is produced when nitrogen and oxygen are combined at high flame temperatures.
These pollutants are increasingly being traced as a major source of respiratory problems in humans and the cause of environmental damage due to the mixture of NOx with moisture which leads to acid rain. Oxygen levels close to Stoichiometric will reduce the availability of oxygen for thermal NOx production and reduce the overall pollution level, with possible reductions of up to 10%.
Although measuring oxygen (excess air) can help match the supply of combustion air to the fuel being burned, maintaining the correct levels can be complicated by a variety of factors that cannot be detected by an oxygen probe alone. These include variations in the fuel and the air, atmospheric pressure and humidity, and the performance of the boiler itself.
Using a combustibles analyser to measure carbon monoxide equivalent (COe) concentrations in the flue gas is an ideal way of compensating for some of these variations. Colourless and highly toxic, carbon monoxide forms when combustibles are not completely consumed. Levels will start to increase when there is insufficient oxygen available to allow complete combustion.
Another possible situation is if the fuel to oxygen mix ratio is incorrect, not all fuel will combust, potentially sending unburned fuel outside the designated “burning areas” of the furnace or boiler. This unreacted fuel mix is also referred to as combustibles or Carbon Monoxide Equivalent (COe), consisting mainly of carbon monoxide, hydrogen and hydrocarbon fuel.
The ultimate goal is to reduce the amounts of combustibles by using the O2 and COe measured values to fine tune this mix. Assuming that we can accurately measure and control both the fuel and air flow into the furnace at the perfect temperature, we can then use the COe value to alarm at the high combustible build up and the potential for exceeding permitted emissions limits.
ABB’s AZ series of oxygen and combustibles analyzers are designed to increase boiler and furnace efficiency and assist in reducing the accumulation of carbon monoxide equivalent gases within the boiler confines.
By effectively measuring COe and O2, plant engineers and operators can also ensure optimal combustion, resulting in reduced fuel costs, maintenance and atmospheric pollutants.